165 



Figure 119: Arrangement of the first dental cones and sphaeridia in the sand dollar, 

 am — ambulacral plate; dc — dental cone; iam — interambulacral plate; sph — sphaeridium. 



As in all echinoderms, the main rudiment from which it develops is the 

 hydrocoel. Long before larval settling, the hydrocoel begins to form the first 

 five processes that will become the radial canals of the ambulacral system of 

 the adult urchin. After this, the hydrocoel bends and takes the shape of a ring, 

 forming the ring canal of the ambulacral system (Figure 123). Soon the 

 coelomic processes are seen to rest upon the bottom of the amniotic sac. In 

 the opinion of McBride (1914), these processes in Echinocardium cordatum 

 are rudiments of the radial canals of the future urchin. A similar picture was 

 observed by Fewkes (1893) during the development of Echinarachnius parma 

 by Aiyar (1936) for Salmacis bicolor (Figure 124), and by other investigators 

 for various urchins. By the time of settling, or soon after it, the first ambu- 

 lacral plates have formed. Aiyar (1936), while describing the metamorphosis 



